1. Field of the Invention
The present invention is in the field of manufacturing tempered glass articles.
2. The Prior Art
As is well known, tempered glass presents certain mechanical advantages over and above conventional annealed glass. Among such advantages there may be mentioned increased mechanical strength and increased resistance to thermal and mechanical shocks. Additionally, tempered glass has the property, when broken, of dicing or falling into a multiplicity of small fragments, the edges of which are relatively dull. As a result of the increased mechanical strength and safety properties of tempered glass, its use has been mandated by many building codes.
Further, it is desirable that lenses, such as eyeglass and camera lenses, be formed of tempered glass, both for purposes of increased ruggedness and to avoid, in the event of breakage, formation of harmful shards or glass fragments.
Notwithstanding the known desirable properties of tempered glass, the employment of structural tempered glass components has been hampered by the difficulty of fabricating the same. Tempering methods heretofore practiced require that the glass element be heated to a temperature somewhere above the annealing point and generally in the range between the annealing and softening points, and thereafter subjected to quenching influences which act in a known manner to effect a condition within the glass wherein shallow zones adjacent the surfaces of the cooled glass are under compression and the internal components of the glass are under tension.
The problem inhering in such method resides in the fact that the heating of glass shapes, and particularly shapes of substantial size, such as an arcuate section of a revolving door, to the temperature range necessary for effecting tempering, renders the glass subject to deformation, whereby the glass will sag or change its shape to a greater or lesser degree, depending upon such factors as the size and curvature of the piece, temperature, manner in which the sheet is supported during tempering, etc.
It will be appreciated that even minor deflections or deformations will render the glass useless for installations wherein the glass must be maintained within reasonably precise tolerance limits.
The tempering of shaped articles without substantial distortion has been effected by fabricating special jigs, molds or like support elements which prevent distortion of the heated article. While it is practicable to fabricate such jigs, molds and the like for high volume production, such as automobile windshields, wherein hundreds of thousands of identical tempered glass elements are to be produced, the cost of forming molds for the production of thousands of units or fewer, renders the procedure prohibitively expensive.
A further difficulty in working with tempered glass is that the glass, after tempering, cannot normally be cut, ground or similarly processed. Thus, optical lenses, the surfaces of which must be formed with precise curvatures, cannot be tempered by conventional methods since the heating and quenching steps would involve sufficient lowering of the glass viscosity to permit slight deformation of shape of the glass. As noted, a lens cannot normally be tempered and then polished to final shape.